and piles of sand, coal, gravel, etc. Table 19 shows the proportion of long loads transmit-
ted to buried pipes. Use the same procedure as in step 7 to compute the load reaching the
buried pipe.
When a sewer pipe is placed on undisturbed ground and covered with fill, compute the
load on the pipe from L = kWd
2
, where d - pipe diameter, ft; other symbols as in step 3.
Tables 18 and 19 are the work of Prof. Anson Marston, Iowa State University.
To find the total load on trenched or surface-level buried pipes subjected to both fill
and long or short loads, add the proportion of the long or short load reaching the pipe to
the load produced by the fill.
Note that sewers may have several cross-sectional shapes—circular, egg, rectangular,
square, etc. The circular sewer is the most common because it has a number of advan-
tages, including economy. Egg-shaped sewers are not as popular as circular and are less
often used today because of their higher costs.
Rectangular and square sewers are often used for storm service. However, their hy-
draulic characteristics are not as desirable as circular sewers.
STORM-SEWER INLET SIZE AND FLOW RATE
What size storm-sewer inlet is required to handle a flow of 2 fWs (0.057 m^3 /s) if the gut-
ter is sloped % in/ft (2.1 cm/m) across the inlet and 0.05 in/ft (0.4 cm/m) along the length
of the inlet? The maximum depth of flow in the gutter is estimated to be 0.2 ft (0.06 m),
and the gutter is depressed 4 in (102 mm) below the normal street level.
Calculation Procedure:- Compute the reciprocal of the gutter transverse slope
The transverse slope of the gutter across the inlet is % in/ft (2.1 cm/m). Expressing the re-
ciprocal of this slope as r, compute the value for this gutter as r = 4 x 12/1 = 48. - Determine the inlet capacity per foot of length
Enter Table 20 at the flow depth of 0.2 ft (0.06 m), and project to the depth of depression
of the gutter of 4 in (102 mm). Opposite this depth, read the inlet capacity per foot of
length as 0.50 ft^3 /s (0.014 m^3 /s). - Compute the required gutter inlet length
The gutter must handle a maximum flow of 2 ft^3 /s (0.057 m^3 /s). Since the inlet has a ca-
pacity of 0.50 ftV(s-ft) [0.047 m^3 /(nrs)] of length, the required length, ft = maximum re-
quired capacity, ft^3 /s/capacity per foot, ft^3 /s - 2.0/0.50 = 4.0 ft (1.2 m). A length of 4.0 ft
(1.2 m) will be satisfactory. Were a length of 4.2 or 4.4 ft (1.28 or 1.34 m) required, a
4.5-ft (1.37-m) long inlet would be chosen. The reasoning behind the choice of a longer
length is that the extra initial investment for the longer length is small compared with the
extra capacity obtained. - Determine how far the water will extend from the curb
Use the relation / = rd, where / = distance water will extend from the curb, ft; d = depth of
water in the gutter at the curb line, ft; other symbols as before. Substituting, we find / =
48(0.2) = 9.6 ft (2.9 m). This distance is acceptable because the water would extend out
this far only during the heaviest storms.
Related Calculations. To compute the flow rate in a gutter, use the relation F =
Q.56(r/n)s°-
5
d
m
, where F = flow rate in gutter, ft
3
/s; n = roughness coefficient, usually